Facebook-only user username override

[nsfx]

I'd like to give "Facebook only" users (users that sign up by clicking Facebook connect) a chance to select a username instead of automatically defaulting to their full name on Facebook. I know this is possible by turning Facebook auto-registration off, but the stakeholders believe the signup form is too daunting (too many fields, too confusing). Is there a hook I can tap into to accomplish this? Thanks.